In normal situations, the surface of the eye, including the sclera, the conjunctiva, and the cornea, is kept moist by the presence of a tear film. This tear film is found in virtually all terrestrial vertebrates, with the exception of snakes.
The surface area of the eye in an adult human is about 2 cm2. It is covered by a complex tear film having a trilaminar structure, with each of the layers having a discrete and necessary function.
Nearest to the surface of the eye is an inner layer of mucus approximately 10 to 20 μm in thickness. The mucus in this layer stabilizes the tear film and provides for attachment of the tear film to the underlying cornea and conjunctiva. The mucus also reduces the surface tension between the tear film and the eye and so permits the tear film to spread evenly across the eye.
The middle layer of the eye is an aqueous layer that is composed largely of water, electrolytes, and various proteins. This layer contains about 2 to 5 μl of aqueous fluid and forms the bulk of the tear film. Within this layer, pH, osmotic pressure, oxygen tension, and the levels of electrolytes such as potassium, calcium, chloride, inorganic phosphates, and acids such as lactic acid and citric acid, are maintained within narrow physiologic ranges. Proteins present in the aqueous layer of the tear film include albumin, and other proteins, such as immunoglobulins, interferon, β-lysin, and lysozyme which have antimicrobial activities.
Farthest from the surface of the eye is a lipid layer, which may range in thickness from a single monolayer to nearly 200 nm. Ordinarily, this layer is about 100 nm thick. This layer serves to retard evaporation of the tear film.
The tear film rapidly decreases in thickness following a blink. Without a subsequent blink, holes will begin to form in the tear film, called tear breakup, within about 30 seconds. Tear breakup times lower than 10 seconds are considered to be abnormal. This can occur with decreased tear formation or deficiencies in the mucus layer of the tear film. Other situations that can result in dryness of the eye surface include environmental aridity, contact lens wearing, and upon waking.
Typically, dryness of the eye is treated with water based solutions containing electrolytes and preservatives which maintain sterility of the solution for multiple applications. Solutions without preservatives are usually packaged in containers that provide for a single use, with disposal of the container and any residual solution following the single application.
The solutions are generally applied by drops, which provide about 20 to 25 μl of fluid to the eye surface. The application of eye drops results in rapid moisturizing of the eye. However, because the amount delivered is greater than the volume of the tear film, these drops have the disadvantage of flooding the eye, which washes away the tear film and replaces the tear film with the fluid that comprises the drops. Immediately following this flooding there exists a period of time when the normal tear film, with its three layer structure and the constituents of each layer, is not present on the eye surface. This can result in incomplete eye moisturizing which lasts for several blink cycles.
Other methods of administration of liquids onto the surface of the eye include eye cups, aerosol and pump sprays, and misters. Eye cups are used to bathe the surface of the eye in fluid, which results in flooding and washing away the tear film that is present on the eye surface. A mister that can be used to deliver a spray of droplets to the eye is described in Hahn, U.S. Pat. Nos. 5,346,132 and 5,893,515, each of which is incorporated herein by reference. In these patents, Hahn discloses several disadvantages of delivering fluid to the eye by drops, including difficulty in positioning the dropper and incomplete delivery of medications due to missing the eye and spilling onto the face. Hahn does not address the issue of the quantity of fluid that is administered to the eye or the issue of washing away the tear film due to flooding. The mister of Hahn delivers a measurable quantity of fluid and can be used for household or medical purposes or to moisturize the eyes or the skin.
Another mister is described in Hutson, U.S. Pat. No. 5,588,564, incorporated herein by reference. Like the mister of Hahn, the mister of Hutson can be used to deliver an adjustable and repeatable dose of fluid to the surface of the eye. Hutson does not address the issue of the quantity of fluid that is administered to the eye or the issue of washing away the tear film due to flooding.
A need exists for a method to moisturize the surface of the eye without flooding the eye or destroying the integrity of the natural tear film.